Lab #3 – Forced Vibration
Short Form Report
Name: ____________________________
Date: _____________________________
Section / Group: ___________________
Procedure Steps (from lab manual):
a. Follow the Start-Up Procedure in the laboratory manual. Note the safety rules.
b. Locate the various springs and masses for the mass-spring-dashpot experimental system.
Part I. Forced Vibrations
Experiment A
Q1. (5%) The apparatus has four position sensors. The driver, encoder 1, encoder 2,
encoder 3. Your prelab used variables 𝑥, 𝑦, and 𝑧 to describe the motion of a system
under base excitation. Fill out the table below to discuss which variables are measured
by which encoder (you may need to write an equation). Also determine which
measurements / variables are absolute and which are relative.
Model Variable
Measurement encoder(s)
Relative or absolute
𝑥
𝑦
𝑧
Q2. (5%)
What is the difference between variables 𝑋 and 𝑥?
Q3. (10%) Your pre-lab discusses two values 𝑟 and 𝜁. In your own words describe what
these values represent. Also, use standard system values (𝑚, 𝑐, 𝑘, 𝑤, 𝑤0 ) to formally
define what these values represent.
Lab #3 Forced Vibrations
Short Form Report
Updated Fall 2015
page 1 of 5
Q4. (5%) Use your data from the previous lab to determine the natural frequency (𝜔0 ) and
critical damping coefficient (𝑐𝑐 ) for cart 2.
Q5. (5%) What frequency range will result in 𝑟 values between 0.6 and 2.0?
When 𝑟 = 0.6, 𝜔 =_______
When 𝑟 = 2.0, 𝜔 =_______
Q6. (5%) What dashpot thumb screw position will result in 𝜁 values between 0.01 and 0.8?
When 𝜁 = 0.01, Position of thumbscrew _________ turns
When 𝜁 = 0.08, Position of thumbscrew _________ turns
We will now study the response of the system to base excitation.
Q7. (10%) Provide sample calculations of 𝑍/𝑌 for two values of r and compare these
results to the values in the spreadsheet.
Lab #3 Forced Vibrations
Short Form Report
Updated Fall 2015
page 2 of 5
Q8. (5%) Use Excel to plot the experimental and theoretical response curves verses r. Print
the plot and label it “Figure 3.A4”. On the same plot, sketch what you would expect
the theoretical response to be for a damping ratio around 0.12.
Q9. (10%) For a damping ratio to a value near 0.08 (note the actual value in the space
below). Calculate the theoretical value of the phase angle, f , for values of 𝑟 close to
0.8, 1.0, and 2.0.
z = _________
fr <1 =
____________
fr » 1 = ____________
fr >1 = ____________
Show Calculations:
Lab #3 Forced Vibrations
Short Form Report
Updated Fall 2015
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Q10. (10%) In order to better understand the meaning of the phase angle, sketch
a(t) 1.0sin(t) and b(t)  2.0sin(t   / 4) ; and clearly mark the phase angle  / 4
on your sketch.
Q11. (15%) We will now estimate the phase angle between the base excitation and the
relative system response. Using figures 3.A1-A3, estimate the phase angle, f , (to
within 15 degrees or 𝜋/12 rads.) for each case. Compare these to the theoretical
values computed in question #8.
Theoretical
Measured
f1 = ____________
f1 = ____________
f 2 = ____________
f 2 = ____________
f 3 = ____________
f 3 = ____________
Lab #3 Forced Vibrations
Short Form Report
Updated Fall 2015
page 4 of 5
Part I. Forced Vibrations
Experiment B
Q12. (10%) Compare the frequency response and phase diagram in Figure 3.B2 with your
results in questions Q7 and Q10. Explain any discrepancies between the plots.
Part II. General
Q13. (5%) Discuss some possible applications of single degree of freedom forced
vibration.
Lab #3 Forced Vibrations
Short Form Report
Updated Fall 2015
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